The invention relates to a circuit configuration for demodulating a voltage that is ASK (amplitude-shift keying) modulated by altering the amplitudes between a low level and a high level.
When using contactless chip cards and the like, such as xe2x80x9ccontactless tagsxe2x80x9d, xe2x80x9cASK modulationxe2x80x9d is often used. This is understood to mean a high-frequency signal that alternates between a first level and a second level using data available in digital form, and thus modulates the high-frequency signal.
In the same way as a distinction is drawn between xe2x80x9cyesxe2x80x9d and xe2x80x9cnoxe2x80x9d or xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d or xe2x80x9chighxe2x80x9d and xe2x80x9clowxe2x80x9d for digital data, a distinction is drawn between a high amplitude and a low amplitude. In this context, two modulation types ASK 100 and ASK 10 are currently the norm. Modulation type ASK 100 signifies a level difference of 100% and ASK 10 signifies a level difference of 10%. Other differences are also possible, however, and the invention described below is not restricted to these two customary modulation types.
The problem with ASK modulation is that when the distance between the sender and the receiver of a signal being modulated in this way changes while the amplitude of the transmitted signal is constant, the received amplitude at the receiver changes. The same applies if differences arise in the intervening space between the sender and the receiver.
To make matters worse, when using signals which always return to xe2x80x9czeroxe2x80x9d (i.e. the signal returns to xe2x80x9czeroxe2x80x9d between two binary xe2x80x9conesxe2x80x9d), and signals which do not always return to zero, the xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d sequences that are modulated and transferred are of different lengths.
It is accordingly an object of the invention to provide a circuit configuration for demodulating a modulated voltage having an amplitude alternating between a low level and a high level, which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.
In particular, the object of the invention is to provide a demodulator circuit, which reliably identifies the level change between two states during ASK modulation operations, and which has as little complexity as possible.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for demodulating a modulated voltage having an amplitude alternating between a low level and a high level. The circuit configuration includes a high-frequency input, and a rectifier circuit connected downstream of the high-frequency input. The rectifier circuit has an output and an input for obtaining an input voltage. The circuit configuration also includes a first charging circuit for producing a charging voltage and a second charging circuit for producing a charging voltage. The first charging circuit and the second charging circuit are connected in parallel to the output of the rectifier circuit. The circuit configuration also includes a decoupling device for decoupling the charging voltage of the first charging circuit and the charging voltage of the second charging circuit when there is a prescribed ratio between the respective charging voltage and the input voltage for the rectifier circuit. The circuit configuration also includes an evaluation circuit for ascertaining a modulation level from the ratio of the charging voltages.
The specified circuit has the advantage that it is a simple matter to identify the change in the modulation level when comparing the two charging voltages.
In accordance with an added feature of the invention, there is provided a floating current-mirror circuit for the first charging circuit and the second charging circuit.
In accordance with an additional feature of the invention, there is provided a voltage transformer for changing the charging voltage of the first charging circuit and/or the is charging voltage of the second charging circuit.
In accordance with another feature of the invention, there is provided a diode for coupling the first charging circuit and the second charging circuit when there is a predetermined ratio between the charging voltage of the first charging circuit and the charging voltage of the second charging circuit.
In accordance with a further feature of the invention, a voltage on the second charging circuit is converted into two different voltages.
In accordance with a further added feature of the invention, the first charging circuit and the second charging circuit have different discharge times.
In accordance with a further additional feature of the invention, there is provided a charging-current amplification circuit and a changeover apparatus for turning on the charging-current amplification circuit from a prescribed degree of modulation onwards.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in circuit configuration for demodulating a voltage which is ASK modulated by altering the amplitude between a low level and a high level, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.